Bring 3 cups sugar, cream, butter, and baking soda to a boil in a heavy saucepan. Remove from heat, and keep warm.
Sprinkle remaining 3/4 cup sugar in a heavy saucepan. Cook over medium heat, stirring constantly, until sugar melts and syrup is light golden brown. (Sugar will clump before melting.) Gradually pour into whipping cream mixture. (Mixture will bubble.) Stir until smooth.
Cook over medium heat, stirring often, 10 to 12 minutes or until a candy thermometer registers 240° (soft ball stage). Remove from heat. Beat at high speed with an electric mixer until spreading consistency (8 to 10 minutes).